Tooth enamel is subjected to a wide variety of natural and clinical conditions that induce chemical changes in its mineral, but very little is known about the response of the aqueous solution within the enamel to changes in the external variables. We propose to study changes in the compositions of the internal solution as functions of time, depth within the tooth (or within a lesion), composition of the external solution, permselectivity of the tooth surface, and degree of mineralization. The objectives are to learn more about the crucial events that occur within the tooth during caries formation, topical fluoridation, etching of the surface, artificial remineralization, or treatment to dissolve the mineral in dental calculus. The objective is to improve preventive methods and clinical procedures. A tooth section about 750 microns thickness is cemented between two plastic slides leaving the natural surface of the tooth exposed at the edge. Small wells are drilled through the top slide into the enamel at various distances from the edge. The assembly is placed on the surface of the test solution. Evaporation is prevented by a layer of mineral oil on the top. Micro electrodes are inserted into the solutions within the wells to measure activities of ions, or galvanic potential differences between wells or between a given well and the external solution. Samples removed from the wells are analyzed for calcium, phosphate, fluoride or other ions by ultramicroanalytical techniques that have been developed for this purpose.